Nested lists in Python - Mini Project: Build & Apply

Working with Nested Lists in Python

📖 Scenario: You are organizing a small library's book shelves. Each shelf holds a list of book titles. The library has three shelves.

🎯 Goal: You will create a nested list representing the shelves and their books, then count how many books are on each shelf, and finally print the counts.

📋 What You'll Learn

Create a nested list called library_shelves with exactly three inner lists, each containing the exact book titles specified.

Create a list called book_counts to store the number of books on each shelf.

Use a for loop with the variable shelf to iterate over library_shelves.

Append the length of each shelf to book_counts.

Print the book_counts list.

💡 Why This Matters

🌍 Real World

Nested lists help organize grouped data like shelves of books, classrooms of students, or menus with categories.

💼 Career

Understanding nested lists is important for data organization, processing multi-level data, and working with complex datasets in many programming jobs.

1

Create the nested list of shelves

Create a nested list called library_shelves with three inner lists. The first list should have 'The Hobbit' and '1984'. The second list should have 'Pride and Prejudice', 'To Kill a Mockingbird', and 'The Great Gatsby'. The third list should have 'Moby Dick'.

Python

# Create the nested list library_shelves with the exact books
# Your code here

Hint

Remember, a nested list is a list inside another list. Use square brackets [] for each shelf.

2

Create the list to hold book counts

Create an empty list called book_counts to store the number of books on each shelf.

Python

library_shelves = [
    ['The Hobbit', '1984'],
    ['Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby'],
    ['Moby Dick']
]

# Create an empty list called book_counts
# Your code here

Hint

Use [] to create an empty list named book_counts.

3

Count books on each shelf

Use a for loop with the variable shelf to iterate over library_shelves. Inside the loop, append the number of books on the current shelf to the book_counts list using len(shelf).

Python

library_shelves = [
    ['The Hobbit', '1984'],
    ['Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby'],
    ['Moby Dick']
]

book_counts = []

# Use a for loop with variable shelf to iterate over library_shelves
# Append the length of shelf to book_counts
# Your code here

Hint

Use for shelf in library_shelves: to loop. Use len(shelf) to get the number of books.

4

Print the book counts

Write a print statement to display the book_counts list.

Python

library_shelves = [
    ['The Hobbit', '1984'],
    ['Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby'],
    ['Moby Dick']
]

book_counts = []

for shelf in library_shelves:
    book_counts.append(len(shelf))

# Print the book_counts list
# Your code here

Hint

Use print(book_counts) to show the list of counts.

Practice

1. What is a nested list in Python?

easy

A. A list with only one element

B. A list that contains other lists as its elements

C. A list that cannot be changed

D. A list that stores only numbers

Solution

1. Step 1: Understand list elements

   A list can hold many types of elements, including other lists.

2. Step 2: Define nested list

   A nested list is a list where some elements are themselves lists.

3. Final Answer:

   A list that contains other lists as its elements -> Option B

4. Quick Check:

   Nested list = list inside list [OK]

Hint: Think of a list inside another list [OK]

Common Mistakes:

• Confusing nested list with single-level list
• Thinking nested list is immutable
• Assuming nested list holds only numbers

2. Which of the following is the correct syntax to create a nested list with two inner lists?

easy

A. [[1, 2], [3, 4]]

B. [1, 2, 3, 4]

C. [1, [2, 3], 4]

D. [[1, 2], 3, 4]

Solution

1. Step 1: Identify nested list syntax

   A nested list has lists inside the main list, each enclosed in brackets.

2. Step 2: Check options

   [[1, 2], [3, 4]] has two inner lists: [1, 2] and [3, 4], correctly nested.

3. Final Answer:

   [[1, 2], [3, 4]] -> Option A

4. Quick Check:

   Two inner lists = double brackets [OK]

Hint: Look for double brackets for nested lists [OK]

Common Mistakes:

• Using single brackets only
• Mixing elements and lists incorrectly
• Missing commas between inner lists

3. What is the output of the following code?

matrix = [[1, 2], [3, 4]]
print(matrix[1][0])

medium

A. 3

B. 1

C. 4

D. 2

Solution

1. Step 1: Access outer list element

   matrix[1] accesses the second inner list: [3, 4].

2. Step 2: Access inner list element

   matrix[1][0] accesses the first element of [3, 4], which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   matrix[1][0] = 3 [OK]

Hint: First index outer list, second index inner list [OK]

Common Mistakes:

• Mixing up indexes (using [0][1] instead)
• Forgetting zero-based indexing
• Confusing inner and outer list positions

4. Find the error in this code snippet:

data = [[1, 2], [3, 4]]
print(data[2][0])

medium

A. No error, output is 3

B. SyntaxError due to missing brackets

C. TypeError because data is not a list

D. IndexError because data[2] does not exist

Solution

1. Step 1: Check list length

   data has two elements: indexes 0 and 1 only.

2. Step 2: Accessing invalid index

   data[2] tries to access a third element which does not exist, causing IndexError.

3. Final Answer:

   IndexError because data[2] does not exist -> Option D

4. Quick Check:

   Index out of range = IndexError [OK]

Hint: Check list length before accessing index [OK]

Common Mistakes:

• Assuming index 2 exists
• Confusing syntax error with runtime error
• Ignoring zero-based indexing

5. Given a nested list nums = [[1, 2, 3], [4, 5], [6]], which code correctly prints all elements in order?

hard

A. print(nums[0][0], nums[1][1], nums[2][0])

B. for i in range(len(nums)):
print(nums[i])

C. for sublist in nums:
for num in sublist:
print(num)

D. for num in nums:
print(num)

Solution

1. Step 1: Understand nested iteration

   To print all elements, loop over each inner list, then each element inside it.

2. Step 2: Analyze options

   for sublist in nums: for num in sublist: print(num) uses two loops: outer for sublist, inner for num, printing each number.

3. Final Answer:

   for sublist in nums: for num in sublist: print(num) -> Option C

4. Quick Check:

   Nested loops print all elements [OK]

Hint: Use nested loops to access nested list elements [OK]

Common Mistakes:

• Printing inner lists instead of elements
• Using single loop only
• Hardcoding indexes without loops